405. Convert a Number to Hexadecimal

Given an integer, write an algorithm to convert it to hexadecimal. For negative integer, two’s complement method is used.

Note:

1. All letters in hexadecimal (a-f) must be in lowercase.
2. The hexadecimal string must not contain extra leading 0s. If the number is zero, it is represented by a single zero character '0'; otherwise, the first character in the hexadecimal string will not be the zero character.
3. The given number is guaranteed to fit within the range of a 32-bit signed integer.
4. You must not use any method provided by the library which converts/formats the number to hex directly.

Example 1:

Input:
26

Output:
"1a"

Example 2:

Input:
-1

Output:
"ffffffff"

two's complement
可以用bit 操作
对于-1， bit位就是全都是1
我们每次，计算最右边的四位，然后向右移动四位
因此存在负数，因此计算的时候为了知道右边四位的bit情况用 num&15

java 里边 >>, <<是数字shift，>>>是逻辑shfit。没有<<<(因为<<<和<<一样)
>>时候最高位不变

Right arithmetic shift  >>Left arithmetic shift  <<
 Right logical shift >>>

   public String toHex(int num) {  
     if(num == 0) return "0";  
     char[] map = {'0','1','2','3','4','5','6','7','8','9','a','b','c','d','e','f'};  
     String ans = "";  
     while(num != 0){  
       ans = map[(num & 15)]+ans;  
       num >>>= 4;  
     }  
     return ans;  
   }